Hydroxide Salts in the Clouds of Venus: Their Effect on the Sulfur Cycle and Cloud Droplet pH

The depletion of SO 2 and H 2 O in and above the clouds of Venus (45–65 km) cannot be explained by known gas-phase chemistry and the observed composition of the atmosphere. We apply a full-atmosphere model of Venus to investigate three potential explanations for the SO 2 and H 2 O depletion: (1) varying the below-cloud water vapor (H 2 O), (2) varying the below-cloud sulfur dioxide (SO 2 ), and (3) the incorporation of chemical reactions inside the sulfuric acid cloud droplets. We find that increasing the below-cloud H 2 O to explain the SO 2 depletion results in a cloud top that is 20 km too high, above-cloud O 2 three orders of magnitude greater than observational upper limits, and no SO above 80 km. The SO 2 depletion can be explained by decreasing the below-cloud SO 2 to 20 ppm. The depletion of SO 2 in the clouds can also be explained by the SO 2 dissolving into the clouds, if the droplets contain hydroxide salts. These salts buffer the cloud pH. The amount of salts sufficient to explain the SO 2 depletion entails a droplet pH of ∼1 at 50 km. Because sulfuric acid is constantly condensing out into the cloud droplets, there must be a continuous and pervasive flux of salts of ≈10 −13 mol cm −2 s −1 driving the cloud droplet chemistry. An atmospheric probe can test both of these explanations by measuring the pH of the cloud droplets and the concentrations of gas-phase SO 2 below the clouds.